The present invention relates to processes for producing ferrous metal compositions having increased corrosion resistance and the compositions and parts made therefrom. More particularly, the invention relates to the discovery that the introduction of powdered aluminum containing compositions into powder standard ferrous metal compositions results in modified compositions that have increased corrosion resistance.
Iron-chromium-nickel and iron-chromium alloys, specifically in the form of stainless steels, have found widespread use in industry due to the highly desirable mechanical and corrosion properties of stainless steels in comparison with conventional low alloy steels. The addition of substantial quantities of chromium to steels results in the formation of a highly protective chromium oxide layer on the surface of the steel that generally protects the underlying metal from corrosion and also provides an excellent surface finish. The addition of nickel enhances the mechanical properties of stainless steels by promoting an austenitic structure in the alloy.
There are, however, a number of problems associated with the use of chromium and nickel. One problem is that nickel is an expensive alloy element that greatly increases the cost of the steel. Another problem is that the majority of the world production of chromium comes from a small number of foreign sources, which means that the supply of chromium is subject to the uncertainties of foreign markets. Therefore, it would be beneficial to reduce the amount of chromium and nickel used in steels.
The protective chromium oxide layer on stainless steels substantially improves the corrosion resistance of the steels to attack by chloride ions compared to low alloy steels. Because of the low resistance of low alloy steels to chloride attack, stainless steels must be used in applications that do not require the enhanced mechanical properties of stainless steels. However, stainless steels do experience higher corrosion rates in marine and other chloride containing environments and exhibit reduced lifetime corrosion performance.
The corrosion resistance of stainless and low alloy steel parts in chloride containing environments is further diminished when powder metal (P/M) steels are used to form the parts. Powder metals are produced by exposing molten metal to cooling gas(es) and/or liquid(s) in such a way that the molten metal solidifies in a particulate powder. The process of producing the powder is known as atomization. An example of a conventional water atomization process is described in U.S. Pat. No. 2,956,304 issued to Batten. While the formability of powder metal provides increased versatility and allows for the production of machine parts that are not readily cast or machined from wrought metal, the corrosion resistance of powder metal parts is generally substantially lower than cast or wrought metal parts. The lower resistance has been thought to be associated with the increased porosity in the compact, which results in increased surface area exposed to the environment, and also related to the exposed microstructure of the powder metal part. As a result, the market for P/M stainless steel parts is only a fraction of the wrought and cast steel markets.
A variety of different metallurgical and mechanical methods have been developed to improve the corrosion resistance of powder metal stainless and low alloy steels. For instance, in U.S. Pat. Nos. 4,240,831, 4,314,849, 4,331,478 and 4,350,529 issued to Ro et al., the inventors disclose that the production of stainless steel powders using conventional water atomization processes, such as that of Batten, resulted in a powder stainless steel that is enhanced in SiO.sub.2 and depleted in chromium near the surface. The chromium depleted region near the surface of the powder resulted in increased susceptibility of the powder to corrosion. Ro et al. found that chromium depletion at the surface could be prevented in the atomization process if certain metals, "metal modifiers", are added to the molten metal prior to atomization. The metal modifiers were found to decrease the amount of silicon dioxide and increase the amount of chromium at the surface of the atomized alloy. The resultant parts formed from the alloy exhibited an improvement in the corrosion resistance over unmodified alloy parts. Ro found tin to be the preferred metal modifier, although other metals such as aluminum, lead, zinc, magnesium, and rare earth metals, were found to concentrate at the surface during atomization and reduce the surface concentration of silicon dioxide, but to a lesser extent than tin.
In U.S. Pat. No. 4,662,939 (the "'939" patent), Reinshagen disclosed a modified molded stainless steel composition, dubbed "Stainless Steel Plus.TM.", having improved corrosion resistance over the base stainless steel that could be prepared by mixing 8-16% of an alloy powder consisting of 2-30% tin and the remainder being either copper and/or nickel with the stainless steel powder prior to molding. However, in subsequent patents, U.S. Pat. No. 5,529,604 and 5,590,384 (the "'604 and '384" patents, respectively), Reinshagen has indicated that the compositions disclosed in the '939 patent grow upon sintering and, as a result, have had only limited acceptance.
In the '604 and '384 patents, Reinshagen discloses that tin could be alloyed with the stainless steel to produce a tin stainless steel powder, similar to Ro et al., which could then be further combined with the Sn-Cu-Ni powder of the '939 patent to provide modified stainless steel powders, named "Stainless Steel Ultra.TM." by the inventor. Powder metal parts formed by the modified stainless steel powder exhibited improved corrosion resistance over conventional stainless steel powder metal parts and do not swell during sintering like the Stainless Steel Plus.TM. parts. See also Reinshagen and Bockius, "Stainless Steel Based P/M Alloys With Improved Corrosion Resistance", a contribution to the 1995 International Conference on Powder Metallurgy and Particulate Materials, May 14-17, 1995, Seattle, Wash.
Other efforts have focused on providing a more tightly compacted powder metal to achieve properties closer to that of cast and wrought materials. Methods include the use of multiple press/sintering processing, including hot forming of the metal powder, varying the treatment conditions of the powder and incorporating powders having higher iron contents. For example, increasing the sintering temperature to more completely reduce the oxide layers on the atomized metal is suggested in "Improving Corrosion Resistance of Stainless Steel PM Parts" Metal Powder Report, Vol. 46, No. 9, p. 22-3 (September 1991). Similar recommendations are made by Reinshagen and Mason in "Improved Corrosion Resistant Stainless Steel Based P/M Alloys" presented at the 1992 Powder Metallurgy World Congress, June 21-26, San Francisco, Calif.
Despite the aforementioned compositional and process changes, powder metal parts have not achieved corrosion resistance that is comparable to cast and wrought parts. Consequently, the market for powder stainless and low alloy steel parts remains only a small percentage of the market for wrought and cast steel parts. As such, the need exists for powder metal compositions that provide increased corrosion resistance, especially with respect to chloride, for use in powder metal parts.